Green’s Dictionary of Slang

a man on horseback with a woman behind him riding side-saddle.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Rackaback. see Gormagon.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: Rackaback. A gormagon.
1811		Lex. Balatronicum.